A revision of the genus Mecistostethus Marseul (Histeridae, Histerinae, Exosternini)

Abstract We revise the genus Mecistostethus Marseul, sinking the monotypic genus Tarsilister Bruch as a junior synonym. Mecistostethus contains six valid species: Mecistostethus pilifer Marseul, Mecistostethus loretoensis (Bruch), comb. n., Mecistostethus seagorum sp. n., Mecistostethus carltoni sp. n., Mecistostethus marseuli sp. n., and Mecistostethus flechtmanni sp. n. The few existing records show the genus to be widespread in tropical and subtropical South America, from northern Argentina to western Amazonian Ecuador and French Guiana. Only a single host record associates one species with the ant Pachycondyla striata Smith (Formicidae: Ponerinae), but it is possible that related ants host all the species.


Introduction
The genus Mecistostethus Marseul is one of the most extremely modified genera of Exosternini in the Neotropics. In fact, like another recently revised genus, Kaszabister Mazur (Dégallier et al. 2012), Mecistostethus has spent much of its taxonomic history placed in the subfamily Haeteriinae, a group principally composed of highly specialized myrmecophilous and termitophilous inquilines (Mazur 1984, 1997, Helava et al. 1985. Its relationship with Exosternini has been recognized only recently (Mazur 2011), and still remains to be formally supported. Mecistostethus was described for a single species, M. pilifer Marseul from the 'Amazon' region, and it has remained monotypic since description. However, a close relative, Tarsilister loretoensis Bruch, another monotypic genus described in Haeteriinae, has remained unassociated (though their relationship was suggested by Helava et al. 1985). Here we formally synonymize these two genera, and present descriptions of several new species.
The morphology of Mecistostethus (Figs 1, 2) presents some extremely autapomorphic features. Principal among these is the very elongated mesometaventrite (from which the genus name, meaning 'very long chest', is derived). In the front the mesoventrite  is inflated and projects ventrad and anterad, concealing the base of the prosternal keel. Posteriorly the metaventral margin is broadly arcuate, projecting deeply into the first abdominal ventrite ( Fig. 2A). Dorsally, the elytra show a strong medial depression across the posterior half. The body is generally broad, depressed and setose (Fig. 1A). All of these features are very atypical, even for the entire family, and have hindered understanding of relationships. Helava et al. (1985), in their revision of the genera of Haeteriinae, examined only Tarsilister and retained it within the subfamily, although they did resolve it as sister of all other haeteriine genera, citing several characters not shared with Haeteriinae as plesiomorphies (narrow antennal scape, tomentose antennal club, presence of tibial spurs, presence of fully articulated coxites with free styli in the females). Only very recent work comprehensively documenting morphological and molecular diversities of both the Exosternini and Haeteriinae (Dégallier et al. 2011;Caterino, Tishechkin, Dégallier, Gomy, and Mazur, in prep.) has produced sufficient character data to conclusively remove this taxon from Haeteriinae and place it unambiguously into Exosternini.
The habits of Mecistostethus are largely unknown. While the unusual morphology strongly suggests an inquilinous lifestyle, only a single host record supports this. This record comes from Bruch's (1932) description of Tarsilister (now Mecistostethus) loretoensis, in which he reports the collection of the unique type in the larval chamber of a nest of Pachycondyla striata Smith (Hymenoptera: Formicidae: Ponerinae). The small number of additional specimens available for study have been collected by flight interception traps. Species of Ponerinae are relatively uncommon hosts of histerids in the neotropics, so it might be premature to assume they are the hosts of the other known species. But it is possible, and certainly merits further investigation.

Materials and methods
The morphological terminology used is that defined by Wenzel and Dybas (1941), supplemented by Helava et al. (1985), Ôhara (1994) and Lawrence et al. (2011). Following histerid conventions, total body length is measured from the anterior margin of the pronotum to the posterior margin of the elytra (to exclude preservation variability in head and pygidial extension), while width is taken at the widest point, generally near the elytral humeri. Conventional imaging was done using a Visionary Digital's 'Passport' portable imaging system, which incorporates a Canon 7D with MP-E 65mm 1-5× macro zoom lens. Images were stacked using Helicon Focus software (www. heliconsoft.com). SEM imaging was done on a Zeiss EVO 40 scope, and the specimen was sputter coated with gold. Photographs of all type specimens are available through the Encyclopedia of Life (www.eol.org).
Specimens from the following collections were studied:

CHND
The

Diagnosis. The genus
Mecistostethus is easily recognized on the basis of numerous autapomorphies, most significantly the elevation of the mesoventrite as a strongly protruding keel ( Fig. 2A) (as opposed to the typically coplanar meso+metaventrite and prosternum), as well as the posteriorly arcuate margin of the metaventrite, projecting deeply into the 1 st abdominal ventrite ( Fig. 2A). In addition the setose body ( Fig. 1A-B), broadened tibiae (Fig. 2a), convex frons ( Fig. 2B), and elytra which are depressed in the posterior third (Fig. 1B), completely lacking dorsal striae 3-5 and sutural stria combine to make this one of the most easily recognizeable New World Histerinae genera. Description. Size range: Length 1.8-2.7mm; width 1.4-2.2mm; Body shape: Body elongate oval, moderately flattened, rufescent to rufo-piceous, variably microsculptured. Head: Frons strongly convex, with epistoma slightly declivous, disk setose, with or without granulate microsculpture; frontal stria outwardly arcuate and subcarinate when present, absent from some species; subraorbital stria present and continuous with sides of frontal stria; labrum about twice as wide as long, apical margin weakly emarginate; mandibles rather short, lacking subapical teeth; antennal scape elongate, slightly swollen subapically; antennal club oval, tomentose, lacking sutures or distinct annuli, with two small dorsal sensoria near apex of upper surface (Fig. 2C); submentum with sutures weakly impressed, bearing a few setae; mentum flat, nearly twice as broad as long, slightly tapered toward apex, apical margin shallowly emarginate; palpi elongate, with apical palpomeres acuminate. Pronotum: Pronotum with sides rounded, narrowed to apex, anterior emargination simple, prescutellar impression absent; pronotal discal gland openings small, annulate, situated about one-third from anterior margin (just beyond ends of recurved anterior submarginal stria, when present), approximately head-width apart; disk generally with punctures near sides and bearing setae variously arranged; marginal stria complete, free anteriorly, bearing 8-11 setae; lateral submarginal stria forming a shallow depression close to marginal stria; anterior portion of marginal stria continuous with lateral submarginal stria; anterior submarginal stria sometimes present, with ends free and recurved posterolaterally. Elytra: Epipleuron lacking striae; dorsal elytral striae subcarinate and bearing setae; outer subhumeral stria complete, cariniform, forming a lateral elytral margin; inner subhumeral, 1 st and 2 nd dorsal striae more or less complete and convergent to posterolateral corner; other elytral striae absent. Prosternum: Prosternal lobe short, extending to hypomeron, with medial fragments of marginal stria in some; prosternal keel posteriorly emarginate, but covered by strongly produced mesoventral process; striae of prosternal keel present or absent. Mesoventrite: Mesoventrite strongly elevated ( Fig. 2A), subacute anteriorly, projecting over base of prosternum; marginal mesoventral stria complete; mesometaventral stria present or absent. Metaventrite: Posterior margin of metaventrite strongly produced posterad. Abdomen: Abdominal ventrites smooth to faintly punctate; abdominal ventrites 2-5 with stria along posterior margin; propygidium short, flat, with two anteromedial gland openings and lateral marginal striae; pygidium rounded apically, setose, with fine marginal stria (Fig. 2D). Legs: Protrochanter with single seta; protibial margin even, bearing fine marginal spines; protibial spurs present, weak; protarsal setae expanded; male protarsal claws simple; meso-and metatibiae expanded, with even, weakly spinose margins; meso-and metatarsi with numerous ventral setae. Male (Fig. 5): Paired accessory sclerites present, weak and small; 8 th tergite with broad basal and narrower apical emarginations, line of basal membrane attachment complete, just distad basal emargination, ventral apodemes widely separated along midline; 8 th sternite with halves separated, apical guides moderately to strongly developed, narrowed apically; 9 th tergite with strong ventrolateral apodemes, about one-third from apex; spiculum gastrale (S9) rather narrow, only slightly expanded at base, more weakly sclerotized along midline, with deep, narrow apical emargination, apical flanges not strongly developed; 10 th tergite entire, not divided along midline; basal piece slightly elongate, from one-fourth to one-third tegmen length; tegmen narrow, variably expanded to apex, with basolateral carinae converging to delimit a ventral concavity, in some with a thin median keel within this concavity; median lobe from one-fourth to one-half tegmen length. Female: 8 th tergite united, emarginate apically, with secondary apicolateral emarginations; 8 th sternite divided into one central and two lateral plates, the basal baculi separate, articulated with the lateral plates; 9 th sternite present as a median plate, with a sclerotized basal connection to sternite 8; 10 th tergite present, undivided; valviferae enlarged at base, articulated with coxites; coxites about one-half length of valvifers, about twice as long as maximum (basal) width, with strong inner apical tooth and much weaker outer one; gonostylus present, setose; bursa copulatrix lumenous, without sclerites; spermatheca and associated glands not examined.
Distribution. The species are exclusively South American, but with scattered records from a surprising variety of biotopes, including Atlantic forests of Santa Catarina, Brazil, cerrado of Mato Grosso do Sul, lowland Amazonian forest of Ecuador and low to mid-elevations on the Guianan shield of French Guiana.
Distribution. Known from Atlantic forest areas of Santa Catarina, Brazil, and subtropical forests near the Rio Paraná in Misiones province, Argentina.

Mecistostethus seagorum
Distribution. This species is only known from the type locality, in south-central French Guiana.
Distribution. This species is only known from the type locality, in southeastern French Guiana.

Mecistostethus carltoni
Distribution. This species is known only from the type locality, in lowland Amazonian rainforest in eastern Ecuador. SEAG. Thanks to Cécile Guitet (Natural and Cultural Patrimony Service), Frédéric Mortier (Parc Amazonien de Guyane), and Nicolas Surugue for permitting SEAG's collections during the Itoupé field excursion. We would especially like to thank Carlos Flechtmann and his students, Silvia Tanabe, Julius Cerqueira, and Bruno Ferreira for their generosity as hosts during our 2011 visit. This project was supported in part by National Science Foundation grant DEB 0949790 to MSC and AKT.